In recent years, coherent optical systems have come back into interest for the upgrade of transmission links for transmission at bit rates of towards 100 Gbps. As bit rates of 100 Gbps have been demonstrated, research activity is now focusing on the next advancement, to hit rates of 400 Gbps or even 1 Tbps. In the light of such an upgrade, moving from standard single-carrier (SC) transmission schemes to alternative schemes, such as orthogonal frequency division multiplexing (OFDM) has been proposed, since the alternatives may enable easier scaling to higher bit rates. One of the most severe impairment that affects coherent systems employing high-order modulation formats is the presence of phase noise introduced by both transmit and receive lasers.
Laser phase noise is due to the instabilities of optical light sources, whose linewidth can vary from a few hundreds kHz to a few MHz, and it can prevent correct detection of transmitted data. Countermeasures to reduce the sensitivity to phase noise of SC coherent systems with inline dispersion compensation have been proposed in G. Colavolpe et al, “J. Lightwave Tech., vol 27, no. 13, pp 2357-2369, 1 Jul. 2009. In OFDM systems, the effect of phase noise is even more severe and represents a major comparison aspect between OFDM and SC.
The impact of phase noise depends on the laser linewidth, the transmission bit-rate, the modulation format, and on the presence of a strategy to combat phase noise. In addition, for long-memory channels (such as dispersion-uncompensated fiber-optic links), the impact of phase noise also depends on the memory length. In the presence of very limited chromatic dispersion, as in links with inline dispersion compensation, in practice an equivalent phase noise, given by the sum of transmit and receive phase noise, is observed at the receiver. In this case, an SC scheme which employs asynchronous strategies for detection and for the adjustment of the equalizer taps described by G. Colavolpe et al exhibits a very good robustness to phase noise. An OFDM system, provided that short OFDM symbols are employed, exhibits a limited performance degradation for lasers' linewidths of practical interest. When larger OFDM symbols are employed a possible solution to improve the robustness against phase noise is the use of a pilot-tone based compensation method.